The present invention relates to chemical mechanical polishers used for polishing semiconductor wafers in the semiconductor fabrication industry. More particularly, the present invention relates to a new and improved system and process for treating wastewater from a chemical mechanical polisher used in the polishing of semiconductor wafers.
In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shadow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a pneumatically-actuated polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica or aluminum.
A CMP process is frequently used in the planarization of an ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 5000 A and about 10,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holder, which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a liquid polishing slurry consisting mainly of colloidal silica suspended in deionized water or KOH solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
Wastewater from the liquid polishing slurry used in the chemical mechanical polishing process must be properly treated for the removal of copper and other chemicals, as well as slurry particles, from the slurry prior to disposal. A typical conventional wastewater treatment system 10 is shown schematically in FIG. 1. The wastewater treatment system 10 receives the wastewater from a CMP apparatus (not shown) during or after the CMP process. The wastewater treatment system 10 includes one or more wastewater collection tanks 12, each of which receives the wastewater through an inlet header 11 and wastewater inlet line 13. Some of the wastewater effluent from the treatment process is distributed into the inlet header 11 through an effluent return line 31 to dilute the wastewater in the collection tank or tanks 12. The wastewater is distributed from each collection tank 12 through a corresponding wastewater outlet line 14 and valve 16, and into a reaction tank 18 through a reaction tank inlet line 19. Sodium hydroxide (NaOH) base may be distributed into the reaction tank 18 through a base infusion line 20, and sulfuric acid (H2SO4) may be distributed into the reaction tank 18 through an acid infusion line 21, in various proportions to achieve a desired pH of the wastewater in the reaction tank 18. Selected quantities of PAC (polyaluminum chloride) coagulator are further distributed into the reaction tank 18 from a PAC supply 22. In the reaction tank 18, the PAC is rapidly mixed with the wastewater to bind or coagulate with the slurry chemicals in the wastewater and precipitate the chemicals out of solution. A reaction tank outlet line 24 distributes the wastewater, with PAC-bound precipitates, from the reaction tank 18 to a clarifier 25, which separates the PAC-bound precipitate particles from the wastewater and distributes the purified wastewater effluent to an effluent collection tank 27 through a clarifier outlet line 26. The PAC-bound slurry particles form a thick sludge which settles in the bottom of the clarifier 25, and the sludge is periodically removed from the clarifier 25 through a sludge removal line 34. Finally, the wastewater effluent is distributed to an effluent line 30 through an effluent outlet line 28 and typically through a valve or valves 29. Excess acid is removed from the effluent line 30 through an acidic waste drain line 32. Some of the effluent is returned to the inlet header 11 through the effluent return line 31, to dilute incoming wastewater in the collection tank or tanks 12, whereas most of the effluent is distributed through an effluent disposal line 33 to a facility disposal system (not shown).
While the PAC has been shown to adequately coagulate and precipitate out of solution chemicals in wastewater from slurry used in most chemical mechanical polishing applications, PAC has been found to inadequately precipitate chemicals, particularly copper cations, in wastewater from slurry used in copper CMP processes, due to the particular chemicals used in the Cu-CMP polishing slurry. This results in production of a wastewater effluent having a high copper content and poor wastewater quality. Accordingly, a new system and process is needed for properly precipitating slurry chemicals, particularly copper cations, in CMP wastewater for the proper treatment and disposal of the wastewater.
An object of the present invention is to provide a new and improved process for treating CMP wastewater.
Another object of the present invention is to provide a new and improved process which effectively removes slurry chemicals from CMP wastewater in the treatment and disposal of the wastewater.
Still another object of the present invention is to provide a new and improved system and process for treating CMP wastewater in a variety of CMP applications.
A still further object of the present invention is to provide a new and improved system and process which is effective in treating wastewater from a copper CMP process.
Yet another object of the present invention is to provide a process which utilizes FSC polymer as a coagulant to remove slurry chemicals from CMP wastewater.
Still another object of the present invention is to provide a system and process which mixes CMP wastewater effluent with FSC polymer coagulant to remove slurry chemicals from CMP wastewater.
Yet another object of the present invention is to provide a CMP wastewater treatment system which includes a sludge return line for returning sludge removed from CMP wastewater in the a clarifier to wastewater in the clarifier in order to utilize the returned sludge as a coagulator for the removal of inert particles from the wastewater.
In accordance with these and other objects and advantages, the present invention is generally directed to a system and process for the treatment of CU-CMP wastewater, including wastewater from a copper CMP process. In a preferred embodiment, the wastewater treatment system includes a coagulant supply tank from which an FSC polymer coagulant is directed into a reaction tank that separately receives the untreated wastewater. The coagulant may first be mixed with the untreated wastewater in selected ratios to provide a desired dosing quantity of the coagulant in the reaction tank. Accordingly, as the wastewater and the FSC polymer coagulant are vigorously mixed in the reaction tank, the coagulant flocs the slurry chemicals, particularly the copper cations, in the wastewater and effectively removes the chemicals from solution in the wastewater as a precipitate before the wastewater is directed to a clarifier. The clarifier separates the flocked precipitate from the wastewater, and the flocked particles settle on the bottom of the clarifier to form a sludge. Some of the sludge is redistributed back into the clarifier to coagulate inert particles in the wastewater. The result is a wastewater effluent which leaves the clarifier with a low copper content and high wastewater quality.